Although confusing and a bit non-standard, Radiance doesn’t use watts/m2.nm for irradiance. Rather, it produces values that if averaged over all the coefficients gives straight watts/m2. So, if you want to convert to watts/m2.nm, you’ll need to divide by the step size, which is (max_lambda - min_lambda)/N where N is the -cs setting.
We chose to use a coefficient average because it avoids conversions between different sampling rates and ranges.
Does this help?
-Greg